The present invention relates to a system and method for controlling a multi-fuel turbogenerator using a modular control architecture for power and fuel control.
Turbogenerators typically include a permanent magnet generator coupled to a turbine to convert heat energy produced by combustion of a fuel into electrical energy for distribution to a load, such as a utility grid. A compressor, driven by the turbine, provides compressed air which is heated by the exhaust gases of the combustion process in a recuperator (heat exchanger) prior to being combined with the fuel in the combustor.
Low emission combustion systems have been developed which introduce excess air into the combustor to lower the combustion temperature and reduce production of nitrogen oxides. The introduction of excess air increases the air/fuel ratios (AFR) to values which approach the weak extinction limit of the fuel. When operating in this low-emissions mode, the fuel is well mixed with the air prior to ignition to produce a homogenous mixture to sustain lean-burning combustion which has a lower peak temperature than stoichiometric combustion. Operation of the turbogenerator in this premixed mode is designed for high generator electrical loads which have associated higher turbine and compressor speeds. Transitions between a premixed operating mode and diffusion modes which service lower loads with higher AFRs must be carefully controlled to provide sustained stable combustion and avoid flame outs. The transition between operating modes is highly dependent upon the particular fuel which is being utilized. Various fuels may include natural gas, diesel, propane, waste gas, and gasoline, for example, which have very different combustion characteristics.
In one aspect, the present invention provides a method for controlling a turbogenerator comprising supplying fuel having known energy content characteristics and air to the turbogenerator, in a first controller determining an air/fuel ratio required to maintain a selected turbogenerator operating parameter at a selected value, communicating the air/fuel ratio to a second controller, and in the second controller selecting a fuel injector mode of operation based upon the air/fuel ratio to maintain flame stability. The mode of operation may include a premix mode or a pilot mode. The premix mode may include energizing any combination of premix fuel injectors.
The present invention provides a modular control architecture to facilitate control of a low emissions turbogenerator so it may be utilized with a variety of different fuels. While control of the system is preferably separated into physically different electronics components and associated hardware which define a power control subsystem and a fuel control subsystem, alternative embodiments employ a modular control architecture within a single controller. For the discrete controller embodiments, the discrete controllers of the power and fuel subsystems communicate via an external intra-controller bus. In both the discrete controller and integrated controller embodiments, the power controller determines an appropriate fuel command to maintain a predetermined turbine exhaust temperature that is based on the current engine speed. The fuel controller selects an appropriate operating mode for one or more injectors based on the AFR. Operating modes include a low emissions premix mode where fuel is highly mixed with air prior to delivery into the primary combustion zone and a pilot or diffusion mode where fuel is directly delivered via a pilot tube to the primary combustion zone without substantial mixing. The fuel controller determines and controls the quantity of fuel provided to the selected injector(s) based on the fuel command communicated by the power controller and the characteristics of the particular fuel being used, such as energy content. Because the fuel command generated by the power controller is in units independent of the particular fuel being utilized, the power control subsystem may be utilized with various fuel subsystems corresponding to a variety of liquid and/or gaseous fuels. The fuel control subsystem controls a fuel metering device based on energy content of the particular fuel being utilized such that the same fuel metering hardware may be used with a variety of fuels having different energy content.
A fuel controller according to the present invention controls the operating mode (premix or pilot) and the number of injectors operating in pilot or premix mode based on the fuel command and required air/fuel ratio. The required air/fuel ratio may be determined directly based on the required air and fuel determinations, or indirectly based on required power. Transitions between operating modes include corresponding hysteresis bands to eliminate oscillation between adjacent modes. The combustor ignitor can be energized during transitions to improve combustion stability. In one embodiment, an increased quantity of fuel is provided to one or more injectors during transitions between operating modes by temporarily increasing the target turbine exhaust temperature which provides additional combustion stability. Preferably, the fuel subsystem includes three injectors and six operating modes including three pilot or diffusion modes and three premix modes. The pilot modes utilize a single injector for low electrical loads and all three injectors for moderate electrical loads. When operating in the pilot modes, fuel is delivered through a pilot tube of the injectors to the combustion zone. A premix mode is selected to service high electrical loads and includes operating all three injectors such that the fuel mixes with air within the injector prior to delivery to the combustion zone within the combustor.
A modular fuel control subsystem according to the present invention includes an electronic fuel controller, a fuel metering/controlling device such as a proportioning valve or pump, and associated temperature and pressure sensors. In addition, the fuel subsystem may include appropriate switching valves, fuel injectors, fuel manifold, and corresponding fuel lines, all of which may be selected depending upon the particular fuel being utilized.
The present invention provides a number of advantages relative to prior art strategies. For example, the present invention allows individual fuel systems to be changed to a new fuel without a corresponding change to the engine or engine controller. The modular control strategy and architecture facilitates independent development and testing of the power controller and fuel controller. Precise monitoring of AFR assures that the minimum fuel is correctly controlled to provide combustion stability and accurate switching between different modes of injector operation. This provides high efficiency low emissions operation for a variety of fuels.
The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.